U.S. Pat. No. 4,333,546 to Fisher describes inflatable evacuation or escape slides intended principally for off-wing (or “over-wing”) use. Because the wing may block view of the ground or other support surface from within the aircraft, passengers and crew may be unable to ascertain visually whether an off-wing evacuation slide has inflated sufficient for use. As noted in the Fisher patent:                It is . . . important for the flight attendant to know whether or not the slide is properly extended without walking to the edge of the wing and looking down. This is important because the attendant must have this information without leaving the fuselage of the aircraft in order to direct the passengers to the escape slides which are operable.See Fisher, col. 1, 11. 13-19. Accordingly, the escape slides of the Fisher patent include two additional indicator tubes designed to inflate upwardly into the line of sight of a flight attendant positioned at the corresponding fuselage access door. The indicator tubes further may include marking tape wrapped around their uppermost ends to enhance their visibility. See id. at col. 3, 11. 38-61. After presumed inflation of a slide, “[t]he attendant then can look out the access door of the fuselage and by observing the positions of the indicator tubes determine if the slide portion, which is not visible from the fuselage, is in condition for evacuating passengers.” See id. at 11. 64-68 (numerals omitted).        
U.S. Pat. No. 6,443,259 to Oley discloses a different mechanism for indicating readiness for use of an off-wing evacuation slide. Instead of using the two upwardly-inflatable tubes of the Fisher patent, slides of the Oley patent include                a conventional red, octagonal “stop” sign that is releasably mounted to the guard rail. The stop sign is removed (to indicate that it is safe to proceed) when the evacuation slide has properly deployed by means of a [lanyard] connected to [the] toe end of the evacuation slide. As the evacuation slide unfurls, the lanyard is paid out until, at the last stage, when the toe end unfurls the lanyard pulls the stop sign off the guard rail, so that it is no longer visible to a disembarking passenger.See Oley, Abstract, 11. 9-17.        
Such a stop sign of the Oley patent is made of fabric and removably attached to the guard rail by hook-and-loop fasteners. See id., col. 3, 11. 61-64. Full deployment of the corresponding slide “yank[s the sign] off its mountings” and pulls it into a sheath for concealment and stowage. See id., col. 4, 11. 25-33. Alternatively, the stop sign may be replaced by “a permanent sign which is concealed by a cover operated by a lanyard, or a conventional yellow ‘police tape’ stretching across the entrance [of the slide], which is removed by a lanyard.” See id., col. 4, 11. 53-57. The contents of the Fisher and Oley patents are incorporated herein in their entireties by this reference.
As criticized in the Oley patent, the inflatable, upwardly-extending “barber poles” of the Fisher patent are disadvantageous at least because “they use valuable inflation gasses to effect their deployment.” Additionally, they “are not intuitive,” as “self-disembarking passengers will not know to look for the barber poles to determine the status of the evacuation slide and may attempt to exit the plane before the slide is properly deployed.” See id., col. 1, 1. 59 through col. 2, 1. 6. Nor are the stop signs of the Oley patent wholly advantageous, however. Passengers and crew receive no affirmative indication of slide readiness, for example; mere absence of a “stop” sign might not be understood to mean “go,” leading to possible confusion among evacuees as to whether evacuation is yet proper. Additionally, failure of the lanyard to overcome the mechanical strength of the hook-and-loop fasteners, tearing of the fabric of the stop sign, or lack of complete stowage of the stop sign in its sheath may result in continued visibility of some or all of the stop sign even though the corresponding slide is ready for use. This too could confuse passengers and crew, incorrectly inhibiting evacuation when it would be proper to do so.